Stabilized precipitation hardening alloys and method of making



NGV. 9, 1965 ROGEN ET AL 3,216,870

STABILIZED PRECIPITATION HARDENING ALLOYS AND METHOD OF MAKING Filed March lo, 1961 FIG. I

INVENTORS NEIL E. ROGEN LASZLO J. BONIS ATTORNEYS 3,216,870 Patented Nov. 9, 1965 3,216,870 STABILIZED PRECIPITATION HARDENING ALLOYS AND NETHOD OF MAKING Neil E. Rogen, Cambridge, and Laszlo J. Bonis, Brookline, Mass., assignors to Ilikon Corporation, Natick,

Mass., a corporation of Delaware Filed Mar. 10, 1961, Ser. No. 4,74ll Claims. (Cl. 148-142) This invention relates to precipitation hardening alloys, and more particularly, relates to a novel and improved method of treating such alloys so that they will retain their high strength at temperatures higher than heretofore possible.

For the purposes of this invention, precipitation hardening alloys are to be defined as a system comprising a base metal and an alloying element or elements which, when heat-treated at a certain temperature, will go into solid solution and which, when subsequently heat-treated at a lower temperature, will provide a secondary phase, in precipitate particle form, consisting essentially of a com pound of the base metal and the alloying element. Such systems are fairly well known; some of the more common being nickel-chromium-titanium aluminum, aluminumcopper, copper-beryllium, and others having copper, iron or cobalt as base or matrix metals. These alloy systems have the advantage of providing high strengths when exposed to elevated temperatures which may be up to 1650 F. Thus, they are particularly suitable in the manufacture of aircraft jet engine parts, turbines, furnaces, atomic reactors, etc. The principal hardening system in nickelchromium-titanium-aluminum alloys is the precipitation of the (Ni, Al, Ti) inter metallic phase in the form of evenly distributed fine particles. Thus, when such alloys are exposed to temperatures above that which they were age hardened, the precipitate compound begins to go back into solid solution with an attendant reduction in strength. When the alloy is maintained at temperatures equal to or exceeding its solid solution temperature for a sufficient time, all of the precipitate will be eliminated and the strength of the alloy will be reduced very markedly.

It is the primary object of this invention to provide an improved alloy of the type described which will maintain its high strength characteristic even under prolonged exposure to temperatures at least in excess of its precipitation hardening temperatures.

It is another object of this invention to provide a novel and improved method for treating precipitation hardening alloys to provide the improved alloy described.

Other objects will be in part obvious, and in part pointed out more in detail hereinafter.

Briefly, and in one aspect thereof, this invention comprises the solution treating of a billet of age hardenable alloy comprising a matrix metal and at least one alloying element which is oxidizable by an atmosphere having a partial pressure of oxygen which will not result in oxidizing of the matrix metal. After solution treating, the alloy is pulverized into fine particles and then reheated in a controlled oxygen atmosphere to a temperature at which a secondary phase will be formed as a fine precipitate in the matrix material. The controlled oxygen atmosphere should have a partial pressure of oxygen sufiicient to oxidize the surface of the precipitate particles but not the matrix material. The oxygen will penetrate the matrix and oxidize the surface of the precipitated particles. The oxide formed will be more stable than the precipitate alone and will prevent the dissolution of the precipitate particles when the alloy is later heated above its age hardening temperature. For example, where the oxidized alloying element is titanium or aluminum, the oxides thereof are stable up to temperatures approaching the melting point of the metal. Thus, the oxidation of the surface of the precipitate renders the precipitate substantially temperature insensitive, and an age hardenable alloy treated in accordance with this invention will be thus referred to as having a stabilized precipitate. The fine particles of the stabilized alloy may then be formed into billets or the like by suitable powder metallurgytechniques. The stabilizing of the precipitate permits the alloy to be extruded or otherwise hot worked without reductions in the high strength imparted by the precipitate, which strength would normally be lost by reason of the precipitate going back into solution during the hot working. A more detailed understanding of this as well as other aspects of the invention may be had by reference to the following detailed description and accompanying drawings in which:

FIG. 1 is a photomicrograph of nickel-chromiumtitanium-aluminum alloy which has been age hardened and then reheated to the temperature of the original solution treatment; and

FIG. 2 is a photomicrograph of the alloy of FIG. 1 which has been stabilized in accordance with this invention and then reheated to the temperature of the original solution treatment.

With reference to the accompanying photomicrographs, it can be seen that in the case of the conventional age hardened alloy, the precipitate will go back into solid solution when the age hardened alloy is exposed to temperatures equal to that of the original solution treatment. How ever, as shown in FIG. 2, when the precipitate of the alloy is stabilized in accordance with this invention, the precipi tate will not go back into solution but will remain in particulate form, thus resulting in the maintenance of the strength of the alloy When it is exposed to temperatures equal to that of the original solution treatment of the alloy.

A specific example of the stabilizing treatment of this invention will now be described in connection with a nickelchromium-titanium-aluminum system, although it will be understood that the invention is not limited to such a system. The alloy is prepared in a conventional manner and in billet form, although, as will be hereafter described, the ratio of titanium to aluminum is quite important. The alloy is then solution treated at a temperature and for a time suflicient to assure that all of the constituents are in solid solution. The alloy is then quenched. The billet is then reduced to particle form in preparation for the next step.

In accordance with the invention, the pulverized solution treated alloy is then reheated in a controlled oxygen atmosphere, to the temperature at which the 7' phase precipitates. This temperature will, of course, be determined by the analysis of the alloy. During this treatment two processes progress at the same time: (1) The particles precipitate; and (2) as they precipitate the diflfused oxygen penetrates into the matrix and oxidizes the titanium or aluminum rich surfaces of the growing particles. The titanium oxide or aluminum oxide both being stable up to very high temperatures, for example, almost up to their melting points, results in the titanium oxide or aluminum oxide ring preventing the 'y' precipitate from going back into solution if the alloy is subsequently heated to or above the temperature at which the precipitate of the unstabilized alloy would begin to go back into solid solution with the matrix metal or metals.

The oxide forming during the stabilizing treatment has been described in the alternative as either titanium or aluminum oxide as the ratio of titanium to aluminum will determine which is in the excess and will be oxidized. In this connection, and as mentioned above, the ratio of titanium to aluminum is important in that during the heat treatments Ni (Al, Ti) will be formed. If the ratio of titanium to aluminum is such that all or substantially all of these elements go into the compound Ni (Al, Ti),

neither of these elements will be available to the oxygen and the stabilizing treatment will not function. Therefore, in the system described, or any other suitable system, the components, other than the matrix metal, forming the secondary phase should be in a ratio such that any reaction between the components of the alloy will leave any excess of one of them, which excess of materials will be rejected by the exact composition of the precipitate to the precipitate-matrix boundary. In accordance with the invention, this excess of oxidizable metal on the surface of the precipitate is oxidized so that the precipitate will be stabilized. Of course, in order to achieve the desired results, the oxide of the oxidizable alloying element must be stable up to temperatures, at least, exceeding the age hardening temperature of the particular alloy concerned.

In connection with the oxygen atmosphere, it is important to control the partial pressure of the oxygen content. For example, if the partial pressure of the oxygen in the case of a nickel-aluminum system, the aluminum in the precipitate will oxidize at a lower partial pressure of oxygen than will the nickel. Thus, in order to avoid oxidation of the nickel matrix which can prevent diffusion of the oxygen and oxidation of the aluminum, it is important that the partial pressure of the oxygen be sufficiently large to effect oxidation of the aluminum but insufficient to effect any substantial oxidation of the matrix. In order to get complete diffusion and oxidation, the oxygen partial pressure normally should be maintained closer to the value required to oxidize the aluminum and stabilize the precipitate. This is because if the partial pressure is too great, even though less than that required to oxidize the matrix, a subscale may be formed in the matrix and prevent complete oxidation of the surface of the precipitated particles. In this event, at least a portion of the secondary phase precipitate will go back into solution when the alloy is exposed to a temperature in excess of its age hardenable temperature.

After the powdered alloy has been simultaneously age hardened and stabilized, it may be formed into billets, semifinished or finished articles, or other suitable form by conventional powder metallurgy techniques. Billets from such stabilized age hardenable alloys can be extruded, hot worked, etc. utilizing temperatures in excess of their solution treatment temperatures without the normally encountered loss in strength. Also, the end products fabricated from a stabilized age hardenable alloy may be exposed to temperatures far exceeding those at which the alloy can normally be exposed without the usual very substantial loss in strength.

A specific example of a stabilizing treatment of a precipitation hardenable alloy involved an alloy having 2.5% titanium, 1.9% aluminum with the remainder being essentially nickel and chromium in an 80%-20% relationship. The percentages given are weight percentages of the total alloy. The alloy was cut into A x A x /2 inch specimens. The specimens were solution treated at 2000 F. and then quenched. Certain of the specimens were then age hardened for ten hours at 1500 F. in a non-oxidizing atmosphere. Other of the specimens were age hardened for 100 hours at 1500 F. but in the presence of a controlled oxygen atmosphere having a partial pressure of oxygen of 10* atmospheres. The unstabilized and stabilized specimens were then again heated to 2000" F. FIG. 1 of the drawings is a 2000 photomicrograph of a portion of one of the solution treated unstabilized specimens which has been etched. As can be seen, the secondary phase precipitate has gone back into solution as would be expected. FIG. 2 of the drawings is a 2000 photomicrograph of a corresponding portion of a stabilized specimen after heating to 2000 F. The specimen of FIG. 2 was also etched in the same manner as the specimen of FIG. 1. Here it can be seen that the secondary plane precipitate is still in particle form and has not gone back into solution. The length of time in the stabilizing step will vary with the rise of the material being treated. For example, while 100 hours were utilized in connection with the above specimens, 5060 hours might be used in connection with 100 mesh powdered alloy.

This invention has been described primarily in connection with the stabilizing of powdered metal. As will be apparent, where stabilized powdered precipitation hardenable alloys are used to fabricate larger articles, all of the metal of the end product will be stabilized. However, where only the surface stabilizing of a larger article is desired or where complete stabilizing of the alloy is not necessary, the treatment of this invention may also be utilized. Further the invention has been described in connection with stabilizing the alloy during the age hardening treatment thereof. It is, of course, to be understood that this might not be the original age hardening alloy. In other Words, existing alloys might be stabilized by again solution treating the alloy, pulverizing the alloy (unless only surface treatment is desired), and then simultaneously age hardening and stabilizing the alloy. In this connection, where it is desired to stabilize the surface of certain alloys which have been age hardened and which have an excess of an oxidizable alloying element on the surface of the secondary phase precipitate, the stabilizing might be accomplished merely by reheating the alloy to its age hardening temperature, and while it is at that temperature, contacting it with an oxygen atmosphere having a suitable partial pressure of oxygen.

It should be noted here that it may be desirable in certain instances to age harden the alloy initially in a neutral or reducing atmosphere and then sweep the furnace with a suitable oxygen atmosphere to accomplish the desired stabilizing. In stabilizing nickel-chIome-titaniumaluminum alloys by both methods, no significant difference in results were noted. However, in other systems it might prove to be advantageous to provide the two step age hardening-stabilizing treatment, and in that event such a treatment should be considered as coming within the scope of this invention.

Also, while this invention has been described in connection with age hardenable nickel-chrome-titaniumaluminum alloys, it is also applicable to other systems containing an oxidizable alloying element and which after suitable heat treatment will provide a secondary phase in precipitate particle form consisting essentially of a compound of the base material and alloying element, with the oxidizable alloying element being on the surface of the precipitate in oxidizable form. The oxidizing of the oxidizable alloying element has been specifically described as being accomplished through the use of an oxygen atmosphere. However, it should be understood that any other suitable means of accomplishing the desired oxidation of the surface of the precipitate is fully within the general scope of this invention. For example, the oxidation might be accomplished by initially compounding the alloy with an oxide which will be stable and will not react with any of the alloy constituents until heated during the stabilizing treatment, whereupon it will decompose and provide free oxygen which will combine with the oxidizable alloying element. In this connection, during the formation of the precipitate, there will be no significant amount of the oxidizable alloy on the surface of the particles until the particles have reached a size consistent with providing increased strength to the alloy. Thus, even though the oxygen may be present from the beginning of the age hardening step, no significant stabilizing of the precipitate will occur until the particles have reached a size which will impart the desired increase in strength to the alloy. An advantage of providing the oxygen by means of an oxide compounded into the alloy is that the stabilizing of the alloy may be accomplished while the alloy is in the form of a billet, thus eliminating the necessity for reducing the alloy to powder prior to stabilizing. As mentioned above, the increased strength obtained with precipitation hardenable alloys is due to the presence of the precipitate in particle form. Stated another way, the precipitate is a foreign body in the matrix, and as long as such foreign bodies in particle form are present, the desired increase in strength will be obtained. Thus, even if the inner portion of a precipitate particle, which has been stabilized according to this invention, should go back into solution, the oxidized surface of the particle will remain in the matrix as a shell or foreign body thus providing a retention of the high strength characteristics of the alloy. The specific examples of this invention which have been given above have been concerned with providing a stable foreign body in the matrix by oxidizing the surface of the precipitate. The oxidation is accomplished by reacting an alloying element with oxygen to provide a stable compound which will not decompose or otherwise break down when the alloy is heated to a temperature in excess of its age hardening temperature. However, it is within the scope of this invention to provide the desired result by reacting an alloying element on the surface of the precipitate with a reactant other than oxygen to provide a stable compound which will remain present in the matrix at temperatures in excess of the age hardening temperature of the alloy. This might be accomplished by compounding the alloy to provide in the alloy a compound which will decompose during heating of the alloy and which has, as an element of decomposition, a reactant which will combine with an excess of alloying element on the surface of the secondary phase precipitate to provide the desired stable compound.

Having thus described our invention, We claim:

1. A method for stabilizing a precipitation hardenable alloy of a base metal and at least one alloying element which will on age hardening provide a secondary phase in precipitate particle form; said method comprising compounding the alloy so that there will be an excess of said alloying element on the surface of the precipitate of said secondary phase, solution treating the alloy, age hardening the alloy, and during said age hardening contacting said alloying element on the surface of the precipitate with a reactant which will combine with said alloying element to provide a compound which is stable at temperatures in excess of the age hardening temperature of the alloy thereby enhancing the stability of the precipitate at these temperatures.

2. A method of providing a stabilized precipitation hardenable alloy of a base metal and at least one alloying element which will during age hardening provide a secondary phase in precipitate particle form; said method comprising compounding the alloy to provide an excess of said alloying element on the surface of the secondary phase percipitate and to provide in the alloy a compound which will decompose on heating and which has as an element of decomposition a reactant which will combine with said alloying element on the surface of the precipitate at elevated temperatures to provide a compound which is stable at temperatures in excess of the age hardening temperature of the alloy, solution treating the alloy, and age hardening the alloy to provide a secondary phase in which the surfaces of the precipitate particles comprise a substantial amount of a stable compound of said alloying element and said reactant thereby enhancing the stability of the precipitate particles at temperatures in excess of the age hardening temperature of the alloy.

3. In a process for providing a precipitation hardenable alloy including a base metal and at least one oxidizable alloying element and which Will during heat treatment of the alloy provide a secondary phase in precipitate particle form consisting essentially of a compound of the base metal and alloying element with an excess of said alloying element on the surfaces of the particles; the improvement of oxidizing the alloying element present on the surface of the precipitate particles of said secondary phase, the oxidizing being conducted while the alloy is at its age hardening temperature and effecting a stabilizing of the percipitate to tend to prevent its going back into solution when the alloy is reheated to a temperature in excess of that at which the precipitate was initially formed.

4. In a process for providing an age hardenable alloy including a base metal and at least one oxidizable alloying element the oxide of which is stable above the age hardening temperature of the alloy, said alloy during suitable heat treatment providing a secondary phase in precipitate particle form with an excess of said alloying element being present on the surfaces of the precipitate particles: the improvement comprising oxidizing the oxidizable alloying element on the surface of the secondary phase prepicitate particles formed by contacting the alloy, during heat treatment thereof to form said secondary phase, with an atmosphere containing oxygen at a partial pressure sufiicient to oxidize the alloying element in the precipitate but insufiicient to oxidize the base metal.

5. In a process described in claim 4: initially conducting the age hardening of the alloy in an atmosphere which is non-oxidizing with respect to said oxidizable alloying element; and, after the particle size of the secondary phase has reached a predetermined size, bringing the oxygen atmosphere into contact with the alloy to stabilize the same.

6. A- method of stabilizing a precipitation hardenable alloy of a base metal and at least one oxidizable alloying element the oxide of which is stable at temperatures at least in excess of the precipitation hardening temperature of the alloy, said alloy providing during age hardening thereof a secondary phase in precipitate particle form with an excess of said alloying element on the surfaces of the precipitate particles: said method comprising contacting the alloy during age hardening thereof with an atmosphere containing oxygen at a partial pressure selected to provide oxidation of the alloying element on the surfaces of the precipitate particles formed during the age hardening of the alloy, the partial pressure of the oxygen being insufiicient to effect oxidation of the base metal to any substantial amount.

7. A method of stabilizing a precipitation hardenable alloy of at least one matrix metal and at least two alloying elements, said alloy providing a secondary phase in precipitate particle form during age hardening heat treatment thereof, one of said alloying elements being oxidizable and the oxide of said alloying element being stable at temperatures in excess of the age hardening temperature of the alloy: said method comprising the steps of compounding the alloy with the ratio of said two alloying elements being such that during the formation of the secondary phase said one of said alloying elements will be rejected by the exact composition of the secondary phase to provide an excess of the oxidizable alloying element on the surface of the particles; and contacting the alloy, while at age hardening temperature, with an atmosphere containing oxygen at a partial pressure suflicient to oxidize said oxidizable alloying element but insufficient to oxidize the matrix metal.

8. A method of providing a stabilized precipitate hardenable alloy of a base metal and at least two alloying elements, said alloy providing during age hardening a secondary phase which is in precipitate particle form and which consists essentially of a compound of the alloying elements and the base metal, at least one of said alloying elements being oxidizable: said method comprising the steps of compounding the alloy with the ratio of said two alloying elements being such that during the formation of the secondary phase said one of said alloying elements will be rejected by the exact composition of the secondary phase to provide an excess of the oxidizable alloying element on the surface of the particles; solution treating the alloy, age hardening the alloy, and during the age hardening step contacting the alloy with sufficient to oxidize said one of the alloying elements but insufiicient to oxidize the matrix material.

9. A method of providing a stabilized age hardened alloy of a base metal and at least two alloying elements which will during age hardening provide a secondary phase in precipitate particle form, said precipitate consisting essentially of a compound of the alloying elements and base metal, one alloying element being oxidizable and being present on the surface of the precipitate particles in oxidizable form, the oxide of said one alloying element being stable at temperatures at least in excess of the age hardening temperature of the alloy: said method comprising providing the alloy in billet form with said alloying elements being in a ratio such that during age hardening of the alloy said one alloying element will be rejected by the exact composition of the secondary phase to provide an oxidizing excess of said one alloying element on the surface of the precipitate particle, solution treating the alloy; reducing the alloy to a powder form, age hardening the powdered alloy in an atmosphere which has a partial pressure of oxygen sufiicient to oxidize said one alloying element but not said base metal, and forming the powdered stabilized alloy in a larger mass.

10. In a method of manufacturing a article of improved age hardened alloy, the steps comprising providing in powdered form a solution treated age hardenable alloy which during age hardening thereof will provide a secondary phase in precipitate particle form consisting essentially of a compound of a base metal and at least one alloying element which is present on the surface of the precipitate particles in oxidizable form, age hardening the alloy, contacting the alloy during age hardening thereof with an atmosphere having a partial pressure of oxygen suflicient to oxidize said one alloying element but not said base metal, and forming the powdered stabilized alloy into a larger mass.

11. In a method of manufacturing an article of improved age hardened alloy, the steps comprising providing in powdered form a solution treated age hardenable alloy which during age hardening thereof will provide a secondary phase in precipitate particle form consisting essentially a compound of a base metal and at least one alloying element which is present on the surface of the precipitate particles in oxidizable form, age hardening the alloy with the entire age hardening step being conducted while the alloy is in contact with an atmosphere having a partial pressure of oxygen sufficient to oxidize said one alloying element but not said base metal, and r forming the powdered stabilized alloy into a larger mass. 12. As an article of manufacture, an improved age hardened alloy having a secondary phase in precipitate particle form consisting essentially of a compound of a base metal and at least one alloying element with said one alloying element being present on the surface of the particles in its oxide form, the oxide of said one alloying element being stable at temperature at least in excess of the age hardening temperature of the alloy and remaining present in the alloy when the alloy is reheated to a temperature in excess of its age hardening temperature.

13. As an article of manufacture, a powdered alloy having a secondary phase in precipitate particle form consisting essentially of a compound of a base metal and at least one alloying element with said one alloying element being present on the surface of the particles in a substantial amount and in its oxide form, the oxide of said one alloying element being stable at temperatures at least in excess of the age hardening temperature of the alloy and remaining present in the alloy when the alloy is reheated to a temperature in excess of its age hardening temperature.

14. As an article of manufacture, a billet and the like fabricated from a powdered age hardenable alloy having a secondary phase in precipitate particle form consisting essentially of a compound of a base metal and at least one alloying element with said one alloying element being present on the surface of the precipitate particles in an oxide form, the oxide of said one alloying element being stable at temperatures at least in excess of the age hardening temperature of the alloy and remaining pres ent in the alloy when the alloy is reheated to a temperature in excess of its age hardening temperature.

15. A method as defined by claim 2 wherein said compound provided in the alloy is an oxide which reacts with the alloying element forming the secondary phase precipitate particle during the age hardening step.

References Cited by the Examiner UNITED STATES PATENTS 2,166,794 7/39 Brophy 75-453 2,283,675 5/42 Harrington 14813.2

FOREIGN PATENTS 654,962 7/51 Great Britain.

OTHER REFERENCES Progress In Metal Physics, vol. 4, 1953, Bruce Chalmers, editor. Pages 151-157.

HYLAND BIZOT, Primal Examiner.

MARCUS U. LYONS, RAY K. WINDHAM, Examiners. 

1. A METHOD FOR STABILIZING A PRECIPITATION HARDENABLE ALLOY OF A BASE METAL AND AT LEAST ONE ALLOYING ELEMENT WHICH WILL ON AGE HARDENING PROVIDE A SECONDARY PHASE IN PRECIPITATE PARTICLE FORM; SAID METHOD COMPRISING COMPOUNDING THE ALLOY SO THAT THERE WILL BE AN EXCESS OF SAID ALLOYING ELEMENT ON THE SURFACE OF HTE PRECIPITATE OF SAID SECONDARY PHASE, SOLUTION TREATING THE ALLOY, ATE HARDENING THE ALLOY, AND DURING SAID AGE HARDENING CONTACTING SAID ALLOYING ELEMENT ON THE SURFACE OF THE PRECIPITATE WITH A REACTANT WHICH WILL COMBINE WITH SAID ALLOYING ELEMENT TO PROVIDE A COMPOUND WHICH IS STABEL AT TEMPERATURES IN EXCESS OF THE AGE HARDENING TEMPERATURE OF THE ALLOY THEREBY ENHANCING THE STABILITY OF THE PRECIPITATE AT THESE TEMPERATURES. 